The present invention relates to percussive rock drill bits, such as used in percussive down-the-hole hammers and top-hammer drills, for example, which bits comprise a steel body carrying hard button inserts. The body includes a plurality of wings. In particular, the invention relates to such drill bits wherein only a single gauge button is disposed in each wing of the drill bit.
Such a drill bit 10 is depicted in FIG. 1 (and also in U.S. Pat. No. 5,025,875). The drill bit includes a steel bit body 12 forming a front cutting face that is intersected by two fluid passages 14 for conducting a flushing/cooling medium (e.g., air) to the front cutting face. Each passage 14 communicates with a radial channel 16 in the cutting face for conducting the flushing/cooling medium to an outer periphery of the bit body, in particular, to respective longitudinal flushing grooves 18' formed in the bit body for facilitating the longitudinal flow of the medium. Additional longitudinal flushing grooves 18 are provided, all of the flushing channels 18, 18' being in circumferentially spaced relationship with one another. Disposed between successive flushing channels in the circumferential direction are so-called wings 20, 20' of the bit body, wherein the wings 20' are situated immediately behind the flushing grooves 18' with reference to the rotation direction d.
Mounted in longitudinal holes formed in the front cutting face are button inserts 22, 23, 23' formed of a hard material such as cemented carbide, especially tungsten carbide. The inserts 23, 23' are mounted in the wings 20, and are known as gauge inserts because they define the diameter of the hole being cut. Two of the gauge inserts 23' are mounted in the wings 20' that are situated immediately behind the flushing grooves 18'. The remaining gauge inserts 23 are mounted in the other wings 20.
FIG. 1 depicts a type of drill bit wherein only a single gauge insert 23, 23' is disposed in each wing 20, 20' (other conventional types of drill bits may have more than one gauge insert in each wing, but the present invention does not pertain thereto.) Typically, in such a drill bit, each gauge insert is located at the center of the respective wing, with reference to a circumferential dimension of the wing. In other words, if the direction of rotation of the drill bit is in the direction d, then the distance A from the center of the gauge insert 23, 23' to a leading edge 24 of the wing 20, 20' is equal to the distance B from the insert center to the trailing edge 26 of the wing, this being the case for all of the gauge inserts.
During rotation of the drill bit, the leading portion of each wing, i.e., the leading edge 24 of the wing, is more exposed than is the trailing edge 26. Consequently, the leading edge wears more rapidly, sometimes limiting the life of the bit. This can happen while the trailing edge remains relatively intact. That is especially true of the wings that are situated immediately downstream of the flushing grooves 18' disposed at radially outer ends of the radial channels 16.
It has been proposed in the prior art to deal with the above-described wear problem by making the flushing grooves narrower, thus extending the total wing length and the total amount of steel. Another solution is to increase the hardness and wear resistance of the steel. Yet another solution is to add wear protection buttons on the outside of the wing. It is not uncommon to put the wear protection buttons on the leading ledge. However, these solutions have drawbacks in regards to flushing capacity of the design, fatigue resistance of the steel and/or manufacturing cost.